Answer:
  a = - 1.8 m / s²
Explanation:
To solve this exercise let's use Newton's second law, let's start by defining a reference system with the x axis parallel to the plane, in the adjoint we can see a diagram of the forces
let's break down the weight
      sin 42 = Wₓ / W
      cos 42 = W_y / W
      Wₓ = W sin 42
      W_y = W cos 42
  Â
Y axis Â
   N- W_y = 0
   N = W_y
   N = mg cos 42
X axis
   fr -Wₓ = m a
   a = (fr - mg sin 42) / m
the friction force has the formula
  fr = μ N
  fr = μ mg cos 42
we substitute
  a = (μ g cos 42– g sin 42)
  a = g (μ cos 42 - sin 42)
 Â
let's calculate
  a = 9.8 (0.650 cos 42 - sin 42)
  a = - 1.8 m / s²
The negative sign indicates that the acceleration is downward, so the boy is descending.
